Dielectric material with non-linear dielectric constant

ABSTRACT

Provided is a composition comprising a polymeric material, a filler material dispersed in the polymeric material, the filler material comprising inorganic particles and a discontinuous arrangement of conductive material wherein at least a portion of the conductive material is in durable electrical contact with the inorganic particles, and conductive material dispersed in the polymeric material.

CROSS REFERENCE To RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.12/962,022, now allowed, which claims the benefit of U.S. ProvisionalPatent Application No. 61/286,247, filed Dec. 14, 2009, the disclosureof which is incorporated by reference herein in its entirety.

TECHNICAL FIELD

This invention relates to a dielectric material having a non-lineardielectric constant and other properties useful for electrical stressrelief.

BACKGROUND

High dielectric constant (Hi-K) elastomeric composites are commonly usedin cable accessories to control electrical field stresses built up atthe locations of splices and terminations. Typically, these materialsare carbon black filled elastomers such as EPDM and silicone that give acertain range of dielectric (K) values for stress relief. Theseelastomeric composites also contain barium titanate (BT) or inorganicfillers that have very high dielectric constants (Hi-K). In order toachieve high dielectric constant of these composites, high fillerloadings (>50 volume percent) are typically required. These highloadings drastically reduce the processability and mechanical propertiesof the resulting composites. For many polymer matrixes, loadings atthese levels are not very practical. For carbon filled composites, thevolume loading of carbon powder should be near the percolation thresholdwhich is very hard to control. For some silicone based systems, Hi-Kpolymeric additives such as epichlorohydrin have been used to increasethe dielectric constant of the resulting composite. These types ofcomposites generally have high dielectric losses (dissipation factor).As a result, such a composite can lead to an increase in temperature inthe dielectric material, which can exceed the thermal load capability ofthe connector and cable.

SUMMARY

One embodiment of the present invention features a novel compositioncomprising: a polymeric material, a filler material dispersed in thepolymeric material, the filler material comprising inorganic particlesand a discontinuous arrangement of conductive material wherein at leasta portion of the conductive material is in durable electrical contactwith the inorganic particles, and conductive material dispersed in thepolymeric material.

Another embodiment of the present invention features a novel articlecomprising: an electrical stress control device comprising a fillermaterial dispersed in a polymeric material, the filler materialcomprising inorganic particles and a discontinuous arrangement ofconductive material wherein at least a portion of the conductivematerial is in durable electrical contact with the inorganic particles,and conductive material dispersed in the polymeric material.

Another embodiment of the present invention features a novel method ofmaking an electrical stress control device comprising:

-   -   forming a filler material comprising inorganic particles and a        discontinuous arrangement of conductive material wherein at        least a portion of the conductive material is in durable        electrical contact with the inorganic particles,    -   blending the filler material into a polymeric material to form a        polymeric composition, and    -   forming the polymeric composition into a stress control device.

As used in this invention:

“electrical contact” between a conductive material and an inorganicparticle means that a portion of the conductive material is touching, oris in sufficient physical proximity to, the inorganic particle so that acharge can travel between the conductive material and the inorganicparticle thereby allowing current to flow directly or by forming anOhmic contact hopping or tunneling effect under an applied voltage fieldof less than the breakdown voltage of the polymeric material;

“durable electrical contact” means that the electrical contact is notsubstantially altered by mixing and shearing forces encountered duringcomposition processing steps; and

“percolation threshold” means the critical fraction of lattice pointsthat must be filled to first create an infinitely continuous conductivepath.

The above summary of the present invention is not intended to describeeach disclosed embodiment or every implementation of the presentinvention. The Figures and detailed description that follow below moreparticularly exemplify illustrative embodiments.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a scanning electron microscope (SEM) digital image of bariumtitanate particles on which carbon powder is affixed according to anembodiment of the present invention.

FIG. 2 is an SEM digital image of a cross-section of a polymericcomposition containing the particles shown in FIG. 1.

FIG. 3 is an SEM digital image of barium titanate particles modifiedwith nanosilica particles according to an embodiment of the presentinvention.

FIG. 4 is an SEM digital image of a cross-section of a polymericcomposition containing the particles shown in FIG. 3.

FIG. 5 illustrates the variation of dielectric constant with electricfield for materials of the invention and comparative materials.

FIG. 6 illustrates the variation of dielectric constant with electricfield for materials of the invention.

FIG. 7 illustrates the variation of dielectric constant with electricfield for materials of the invention and comparative materials.

FIG. 8 illustrates the variation of dielectric constant with electricfield for materials of the invention.

FIG. 9 illustrates the variation of dielectric constant with electricfield at 25 kV for a material of the invention.

DETAILED DESCRIPTION

In the following detailed description of the preferred embodiments,reference is made to the accompanying drawings that form a part hereof.The accompanying drawings show, by way of illustration, specificembodiments in which the invention may be practiced. It is to beunderstood that other embodiments may be used, and structural or logicalchanges may be made without departing from the scope of the presentinvention. The following detailed description, therefore, is not to betaken in a limiting sense, and the scope of the invention is defined bythe appended claims.

Embodiments of the present invention include novel filler materials suchas the one shown in FIG. 1. The filler material includes inorganicparticles on which conductive material, such as conductive particles, isaffixed in durable electrical contact. As will be explained in moredetail later, the conductive material is applied to the inorganicparticles in a manner that provides a sufficient electrical, e.g.,static, or chemical, attraction between the inorganic particles andconductive material to inhibit the conductive material from separatingfrom the inorganic particles during handling and subsequent materialprocessing steps. The inorganic particles with which the conductivematerial is affixed in durable electrical contact may then be added to apolymeric material to form a dielectric composition. These compositionshave significantly better electrical properties than traditional carbonfilled polymers.

In some embodiments, the compositions were first prepared by affixing indurable electrical contact the surface of barium titanate (an inorganicferroelectric ceramic) particles with a highly structured form ofconductive carbon powder that has high void volume and highconductivity, such as that available under the trade designation ENSACO250 G, from TimCal Graphite & Carbon Corp., Bodio, Switzerland, andhaving a nominal particle diameter of 40 nm, and then dispersed in asilicone polymer (a polymer having an SiO backbone) matrix as shown inFIG. 2. The resulting elastomeric compositions after curing had a highdielectric constant (>20), low loss (<0.04) and high dielectricbreakdown strength (>140 V/mil) and unexpectedly exhibited fielddependent permittivity (non-linearity). These non-conducting (low loss)compositions exhibited the unique non-linear property of a graduallyincreasing dielectric constant with an increasing electric field. Insome preferred embodiments, the barium titanate volume loading in thecomposition is greater than 20 volume percent and the barium titanate tocarbon percent volume ratio is between about 6 and about 12. However,the elongation to break for these compositions is less than about 150%,so they are most suitable for applications that do not require superiormechanical properties.

In other embodiment of the invention, good mechanical properties as wellas the unique non-linear electrical property are obtained. In theseembodiments, the composition includes an elastomeric composite comprisedof (a) a high dielectric constant filler such as nanosilica (i.e.,nanometer sized silica particles)-modified barium titanate (25 v %), (b)carbon powder (3.0 v %) and (c) silicone oil (an oil comprisingoligomers having an SiO backbone) (10 v %) in a silicone rubber matrix.The unique combination of nanosilica-modified barium titanate togetherwith the silicone oil additive substantially enhanced the filler (bariumtitanate) dispersion and reinforcement with silicone matrix. As aresult, this composition showed improved mechanical (elongation tobreak>300%, tensile strength 372-520 psi) and electrical (dielectricconstant 23-30, dissipation factor<0.05 and breakdown strength 180-210V/mil) properties, and had a preferred conductivity profile thatprovided an improved impulse performance. These improved properties makeat least some embodiments of the composition and articles of theinvention especially useful for stress control in high voltage cableaccessories that require superior mechanical properties, such ascold-shrink applications.

Some of the improved properties were achieved by improving fillerdispersion and reinforcement with silicone rubber by using a uniquecombination of nanosilica-modified filler (barium titanate) and siliconeoil additive. An example of the nanosilica-modified filler is shown inFIG. 3. The composite showed homogenous particle distribution throughoutthe silicone matrix, as shown in FIG. 4, and also had substantiallyimproved electrical properties as well.

Suitable materials for the inorganic particles of the present inventioninclude, for example, BaTiO3 particles, BaSrTiO3 particles, CaCu3Ti4O12particles (including, e.g., particles calcined or sintered at atemperature of 800° C.), and SrTiO3 particles, or mixtures thereof. Suchparticles may be pure or may modified, such as by doping, or by addingother ingredients. Preferably the inorganic particles have a relativedielectric constant of greater than 80. The inorganic particles may haveany suitable shape such as spheres, plates, platelets, cubes, needles,oblate, spheroids, pyramids, prisms, flakes, rods, fibers, chips,whiskers, etc. or mixtures thereof. A suitable size, e.g., diameter, forthe inorganic particles is lower limit of about 0.7 μm to about 1.0 μm,and an upper limit of about 0.8 μm to about 2.1 μm.

The inventors found that the mechanical properties of at least someembodiments of the compositions of the invention could be enhanced bymodifying the inorganic particles with nano-silica. For example, it wasfound that the combination of nanosilica-modified barium titanate withsilicone oil substantially enhanced the barium titanate dispersion andreinforcement in the matrix of silicone polymer material. The bariumtitanate was modified with nanosilica by mixing the barium titanate withhydrophobically-modified nanoparticles in toluene and evaporating thetoluene. The dried material was shaken with ceramic marbles to reduceparticle agglomeration. The nanosilica-modified barium titanate was thenground together with carbon powder. A suitable weight % of nano-silicaparticles to inorganic particles is about 0.5 to about 1.0, preferablyabout 0.75. Suitable sizes of the nano-silica particles are about 1 toabout 50 nm, preferably about 5 nm. Typically, the inorganic particleson which the nano-silica particles are applied have a diameter of about0.8 μm to about 2.1 μm.

Suitable materials for the conductive material include, for example,carbon blacks, carbon nanotubes, insulating particles having conductivecoatings, metals and metallic powders, for example aluminum, gold,silver, chromium, copper, palladium, nickel and alloys thereof. Theconductive material may be in any suitable form such as clusters, e.g.,clusters of carbon particles, individual particles, and vaporized solidsthat may be coated or deposited on the inorganic particles. If theconductive material is particulate, it may have any suitable shape suchas spheres, plates, platelets, cubes, needles, oblate, spheroids,pyramids, prisms, flakes, rods, fibers, chips, whiskers, etc. ormixtures thereof.

The application, or affixation, of the conductive material to theinorganic particles can be performed in any suitable manner, such as,for example, grinding, ball milling, impact-coating, andmagnetically-assisted impact coating the conductive material andinorganic particles together, coating, solvent-coating,vapor-depositing, and liquid dispersing the conductive material on theinorganic particles, or using any other known suitable method such thatthe conductive material forms a discontinuous arrangement wherein atleast a portion of the conductive material is in durable electricalcontact with the inorganic particles. The conductive materials may beapplied to a small or large area of the surface of the inorganicparticles. Determination of the appropriate amount of conductivematerials applied to the inorganic particles depends on various factorssuch as the combination of materials in the composition, e.g.,conductive material, inorganic particle, polymer, additives, and theintended use of the material.

The basic polymeric material may be selected from a large range ofpolymers. Blends of two or more polymers may be desirable in some casesand the polymers selected will depend at least to a certain extent onthe purpose to which the material is to be put. Examples of polymerssuitable either alone or in blends include elastomeric materials, forexample silicone or EPDM; thermoplastic polymers, for examplepolyethylene or polypropylene; adhesives, for example those based onethylene-vinyl-acetate; thermoplastic elastomers; gels; thermosettingmaterials, for example epoxy resins; or a combination of such materials,including co-polymers, for example a combination of polyisobutylene andamorphous polypropylene, epichlorohydrin polymers, fluoroelastomerpolymers, and blends of epichlorohydrin and fluoroelastomer polymers.

The compositions may also comprise other well-known additives for thosematerials, for example to improve their processability and/orsuitability for particular applications. In the latter respect, forexample, materials for use as power cable accessories may need towithstand outdoor environmental conditions. Suitable additives may thusinclude processing agents, stabilizers, antioxidants and plasticizers,for example oil, such as silicone oil. Compositions of the invention aremade by mixing the inorganic particles on which conductive material isaffixed with the polymer and any desired additives. In many embodimentsof the compositions, conductive material, which is the same or differentas the conductive material coated on the inorganic particles, will bedispersed in the polymeric material.

In at least one embodiment of the invention, the composition includesthe discontinuous arrangement of conductive material on the inorganicparticles in electrical contact with the inorganic particles and furtherincludes conductive material dispersed in the polymeric material. Thetotal amount of conductive material in the composition is between about40 and about 70 vol % of the amount of conductive material needed toattain the composition's percolation threshold.

In at least one embodiment of the invention, the composition has arelative dielectric constant greater than about 15, preferably greaterthan about 18 and a dielectric loss of less than about 0.12, preferablyless than about 0.05.

In at least one embodiment of the invention, the composition has adielectric breakdown strength greater than about 4 kiloVolts/millimeter(kV/mm), preferably greater than about 7.2 kV/mm.

In at least one embodiment of the invention, the composition has arelative dielectric constant value that changes in a non-linear mannerupon a change in applied voltage as illustrated in FIGS. 5 through 9.

In at least one embodiment of the invention, the polymeric material isan elastomeric material and the composition has an elongation at breakof greater than about 150%, preferably greater than about 300% and apermanent set (as per ASTM D 412-06a) of less than about 25, preferablyless than about 20, more preferably less than about 10.

In at least one embodiment of the invention, the composition has amodulus of elasticity of greater than about 150 pounds per square inch,preferably greater than about 230 pounds per square inch, and morepreferably greater than about 300 pounds per square inch .

The compositions of the invention can be used in various articles forvarious applications, e.g., spray, coating, mastics, tapes, and shapedbodies having a definite configuration. The compositions of the presentinvention are particularly suitable for use in stress control elementsor devices such as high voltage cable accessories, wherein the nonlinearproperties of the compositions are useful. Dielectric stress controldevices can be manufactured which are designed with respect to theirdielectric properties and their geometric configurations in accordancewith desirable modifications of an electric field present at therespective site of application. These stress control devices consist atleast partly of the composition of the invention. Particularly useful isa dielectric stress control device or element which consists of a shapedbody, preferably a sleeve, which can be placed onto an end of a cableinsulation and/or shield. Stress control devices or elements havingother geometric configurations may be useful to prevent unacceptablyhigh local field concentrations, for example in break elbows, transitionor throughgoing connections, feed throughs and branchings of hightension cables.

In at least one embodiment, the composition has elastomeric properties.This allows cold-shrink dielectric stress control devices to bemanufactured which are suited for different dimensions or sizes ofelectrical structural components. For example in the case of sleeves,same may have sufficient resilience to be applicable with cableinsulations and/or dimensions of various thicknesses.

The articles of the invention may be used in, for example, the followingapplications:

-   -   (i) Insulation for electric cables, where this insulation is        situated between the conductor and the primary dielectric or        between the screen of the cable and the primary dielectric.    -   (ii) Insulation for electric cables as in the layered        construction described in U.S. Pat. No. 3,666,876.    -   (iii) Stress control coverings for electrical cable        terminations. Such stress control means may be in the form of        sprays, coatings, mastics, molded parts, tubing or tape and may        be used with or without an external protective layer, as        necessary.    -   (iv) Stress control coverings for stator-bar ends or the ends of        insulated electrical conductors, e.g., motor windings, in        machines.    -   (v) Stress control components in lightning arrestors.    -   (vi) As components of insulator bodies where the material may be        the outer layer or an internal component, provided that it is        non-tracking in service; thus it could be used for sheds or        tubing to provide insulators for tension suspension, post or        bushing insulators.

Although specific embodiments have been illustrated and described hereinfor purposes of description of the preferred embodiment, it will beappreciated by those of ordinary skill in the art that a wide variety ofalternate and/or equivalent implementations may be substituted for thespecific embodiments shown and described without departing from thescope of the present invention. This application is intended to coverany adaptations or variations of the preferred embodiments discussedherein. Therefore, it is manifestly intended that this invention belimited only by the claims and the equivalents thereof.

EXAMPLES

The following examples and comparative examples are offered to aid inthe understanding of the present invention and are not to be construedas limiting the scope thereof. Unless otherwise indicated, all parts andpercentages are by weight. The following test methods and protocols wereemployed in the evaluation of the illustrative and comparative examplesthat follow:

Material List

TABLE 1 Ingredient Product Name Source Barium Titanate 219-6A BariumFerro Corporation, Titanate (0.8-2.1 Cleveland, OH micron) Carbon PowderENSACO250 G TimCal Graphite & Carbon (40 nm) Corp., Bodio SwitzerlandCollodial Silica NALCO 2326 Nalco, Bedford Park, IL Isoctyltrimethoxysilane Gelest, Morrisville, PA Methyltrimethoxy silane Gelest,Morrisville, PA Ethanol 80:20 EMD, Gibbstown, NJ Methanol VWR, WestChester, PA Liquid Silicone Rubber ELASTOSIL LR Wacker Chemie AG,3003/30 A/B Munich, Germany Silicone Oil DOW CORNING Dow CorningCorporation, (Polydimethlysiloxane) 200 FLUID Midland, MI SilicaTitanium dioxide Calcium Titanate Alfa Aesar, Ward Hill, MA AluminumPowder Alfa Aesar, Ward Hill, MA Toluene Alfa Aesar, Ward Hill, MA

Test Methodologies

1. Relative dielectric constant and dissipation factor (loss)measurement: ASTM D150-98 (2004)

2. Breakdown strength: ASTM D149-09

3. Non linear relative dielectric constant: ASTM D150-98 (2004) modifiedby changing the voltage source to impulse waveform of 1.2microseconds/50 microseconds.

4. Elongation to break: Standard Test Methods for Vulcanized Rubber andThermoplastic Elastomers—Tension, ASTM D 412-06a Published January, 2007

5. Permanent Set: Permanent Tension Set of Rubber-22 hrs @ 100Celsius—Electrical Products Standard, 3M Test Method TM-86D, Issue Date:Nov. 22, 1994

6. Volume Resistivity (Inverse of Electrical Conductivity): ASTM 257-07.

Example 1 to 5 and Comparative Examples C1 to C5

For examples 1-5, an inorganic filler material was first prepared bydecorating conductive particles onto the surface of an inorganicparticle, in this case a ferroelectric ceramic material. In theseexamples, barium titanate (BT) was used as the inorganic particle(particle size 0.8-2.1 micron), and a highly structured carbon powder(ENSACO 250 G) (C) was used as the conductive material. The carbonpowder was decorated onto the barium titanate particle surface by mixingand pressing or grinding them together in a mortar and pestle for 5-10minutes, until a homogeneous dispersion was obtained (as determined bythe naked eye.). The resulting filler material was then blended in aliquid silicone rubber matrix. The volume percents of the BT and C inthe final mixture and the BT:C ratios for each example are given inTable 2.

The resulting mixture was poured into a mold cavity (100 mil deep and1.25 inch inner diameter) and partially cured at 160° C. for 8 minutesin a press. It was then removed from the mold and further cured in aconvection oven at 200° C. for 4 hours. Electrical properties such asdielectric constant, dissipation factor and dielectric breakdownstrength of these molded disks were then measured at ambient conditions.Example C1 describes a barium titanate (40 volume percent) controlsample without the carbon powder. Examples C2 and C3 describe controlsamples with two filling levels of the carbon powder (3 and 5 volumepercent) without barium titanate. The barium titanate and carbon powderwere each separately blended in liquid silicone rubber using a “speedmixer” available under the trade designation DAC 150FVZ from FlackTek,Inc., Landrum, SC, at 3000 rpm for 30 seconds. The resulting mixture wasmolded in the same manner as Examples 1-5.

In Example C4, the barium titanate and carbon powder were mixed togetherbut with no grinding. In Example C5, carbon was dispersed in a siliconerubber matrix followed by addition of the barium titanate particles. Allof the comparative examples were molded into disks and cured asdescribed for Examples 1-5.

The electrical properties of the resulting molded disks of Examples 1 to5 and Comparative Examples C1 to C5 are listed in Table2.

TABLE 2 Composite Barium Carbon Dielectric Dissipation DielectricTitanate Black BT/C Constant Factor (D) Breakdown Mixing Ex. (v %) (v %)ratio (K) at 100 Hz at 100 Hz Strength Process 1 24.5 3 8.17 24.1 0.03465.79 kV/mm BT/C (147.1 V/mil) Grinding 2 27.5 3 9.12 21.1 0.0165 7.25kV/mm BT/C (184.2 V/mil) Grinding 3 30.0 3 10 21.7 0.0139 7.10 kV/mmBT/C (180.4 V/mil) Grinding 4 20.0 3 6.67 14.5 0.0066 9.05 kV/mm BT/C(230 V/mil) Grinding 5 24.5 2 12.25 9.8 0.0016 11.69 kV/mm BT/C (296.9V/mil) Grinding C1 40.0 0 40 13.5 0.0057 11.58 kV/mm NA (294.2 V/mil) C20.0 3 5.4 0.0020 12.87 kV/mm NA (327 V/mil) C3 0.0 5 188.9 0.6565 3.20kV/mm NA (81.4 V/mil) C4 30.0 3 10 40.6 0.0381 3.84 kV/mm BT/C No (97.6V/mil) Grinding C5 24.5 3 8.17 62.5 0.1415 2.60 kV/mm C dispersion (66V/mil) followed by BT addition

The variation of dielectric constant with electric field (non-linearproperties) on selected examples in table 2 was measured by using thenon-linear relative dielectric test. These test results are shown inFIG. 5. As seen in FIG. 5, Examples 1 and 3 show a non-linear increasein dielectric constant value with electric field increase. Thedielectric constant value increases from 24.1 to 140 in Example 1 andthat increases from 21.7 to 120 in Example 2 as the field strengthincreases up to 5.5 kV/mm. Under those experimental conditions, theComparative Examples C1, C2 and C3 do not show non-linear dielectricproperties.

FIG. 6 shows the dielectric constant data of Examples 1, 3, 4, and 5. Asseen in FIG. 6, Example 4 as well as Examples 1 and 3 show somenon-linear dielectric properties whereas Example 5 shows no non-lineardielectric properties in the range of applied electric field.

As seen in Table 2, both Examples C4 and C5 have lower electricbreakdown strength values than Examples 3 and 1, which have the same BTand C content, respectively. Example C4 has a breakdown strength of 3.84kV/mm (97.6V/mil) and Example C5 has a dielectric breakdown strength of2.60 kV/mm (66V/mil). In addition, the dielectric constants increasemore rapidly with electric field in these examples than for Examples 1and 3. In contrast, Examples 1 and 3 show gradual increase in dielectricconstant and can withstand significantly higher field strength beforereaching the dielectric breakdown of the material (Example 1 dielectricbreakdown is 5.79 kV/mm and Example 3 dielectric breakdown is 7.10kV/mm).

Examples 6-8—Fillers with Different K Values

In these examples, barium titanate particles were substituted withsilica, titanium dioxide calcium titanate, and strontium titanateparticles. Silicone rubber disks were prepared as described for Examples1-5 after grinding 30 volume percent of each type of inorganic particlewith 3 volume percent of carbon powder. Electrical properties of each ofthese disks were then measured. The test results are summarized in Table3, along with the test results for Example 3. In addition, non-lineardielectric properties were measured for Examples 6-8. The test resultsare shown in FIG. 8.

TABLE 3 Inorganic Dielectric particle Composite Breakdown Inorganicdielectric Dielectric Dissipation strength Ex. particle constantConstant factor (D) (V/mil) 6 Silica 3 8.9 0.032 268 7 Titanium dioxide70-80 11.9 0.005 205 8 Calcium Titanate 200-300 18.4 0.014 217 3 BariumTitanate 2000-4000 21.7 0.0139 180.4

Example 9

In this example, carbon powder was substituted with 18 volume percentaluminum powder (10 micron size) (calculated using a density of 1.5g/cc). A silicone rubber disk was prepared as described in Examples 1-5after grinding the Al powder with 24.5 volume percent barium titanate.The resultant disk had a dielectric constant (K) of 20.8 and adissipation factor of 0.022.

Example 10 Preparation of Hydrophobically Modified Nanosilica Particle:

A mixture of 100 grams of colloidal silica (16.06 wt. % solids in water;5 nm size), 7.54 grams of isoctyltrimethoxy silane, 0.81 grams ofmethyltrimethoxysilane and 112.5 grams of an 80:20 wt/wt. % solventblend of ethanol: methanol were added to a 500 ml 3-neck round bottomflask (Ace Glass, Vineland, N.J.). The flask containing the mixture wasplaced in an oil bath set at 80° C. with stirring for 4 hours to preparehydrophobically modified nanosilica particles. The hydrophobicallymodified nanosilica particles were transferred to a crystallizing dishand dried in a convection oven at 150° C. for 2 hours.

Nanosilica Particle Modification of Barium Titanate Filler:

Barium titanate particles (particle size 0.8-2.1 microns) were modifiedby mixing (using a spatula) with the hydrophobically modified nanosilicaparticles (0.75 wt %) and dispersing in excess toluene. The bariumtitanate and nanosilica particle mixture was rolled overnight and thetoluene was then evaporated off at 150° C. The resulting powder wastransferred to a large Nalgene bottle, four large ceramic marbles wereadded to the powder and shaken by hand for several minutes. Thisprocedure resulted in a filler composition that had significantlyreduced particle agglomeration. The scanning electron micrograph (SEM)of the nanosilica particle modified barium titanate is shown in FIG. 3.

Example 11: Preparation of Silicone Rubber Composites

Nanosilica particle modified barium titanate (NS BT) was decorated withcarbon powder as described in Examples 1-5. About 25 volume percent NSBT and 3.0 volume percent carbon powder were ground together with amortar and pestle for 5-10 minutes, until a homogeneous dispersion wasobtained (as determined by the naked eye.). The ground powder mixturewas blended in 62 volume percent liquid silicone rubber and 10 volumepercent silicone oil using a “speed mixer” available under the tradedesignation DAC 150FVZ from FlackTek, Inc., Landrum, SC, at 3000 rpm for30 seconds. The resulting silicone rubber composite was then poured intoa mold (3×6×0.07 in) and partially cured at 160° C. for 10 minutes in apress. The partially cured slab was then removed from the mold andfurther cured at 200° C. for 4 hours. A cross-section SEM of the curedslab shows homogenous distribution of NS BT particles throughout thesilicone matrix (FIG. 4).

Three samples were used for each test conducted to determine electricaland mechanical properties. The ranges of the test results for the threesamples are given below.

Electrical Properties:

Dielectric constant and dissipation factor measurements were made byfollowing the ASTM D150-98 (2004) test procedure at 100 Hz. Volumeresistivity measurements were made by following the ASTM 257-07 testprocedures at 100 Hz. Dielectric breakdown strength measurements weremade by following the ASTM D149-09 test procedure. The range of testresults is as follows:

Dielectric constant 23-30

Dissipation factor<0.05

Volume Resistivity: 1.4 E8-E9 Ohm/m

Dielectric breakdown voltage strength 180-210 V/mil range

The electrical field dependent relative dielectric constant underimpulse condition was measured at 25 kV by using the non-linear relativedielectric constant test. The test results are shown in FIG. 9.

Mechanical Properties:

The tensile strength, percent elongation to break, modulus and permanenttension set are measured by using ASTM D412-06a test procedure. Therange of test results is as follows:

Tensile strength: 372-498 psi

Elongation to break: 320-410%

Modulus: 232-255 psi @ 100% elongation

-   -   285-429 psi @ 200% elongation    -   300-479 psi @ 300% elongation

Permanent tension set 9.4-10.10%

In comparison, to the 320-410% elongation to break of Example 11, theelongation to break of Example 3, made without the NS BT and siliconoil, was 166%.

What is claimed is:
 1. A composition comprising: an elastomericmaterial; a filler material dispersed in the elastomeric material, thefiller material comprising inorganic particles and a discontinuousarrangement of conductive material, wherein at least a portion of theconductive material is in durable electrical contact with the inorganicparticles; and conductive material dispersed in the elastomericmaterial; wherein the volume ratio of the inorganic particles toconductive material is about 6 to about 12; and wherein the inorganicparticles are modified with nanosilica.
 2. The composition of claim 1wherein the conductive material is selected from the group consisting ofcarbon black, carbon nanotubes, clusters of carbon particles, graphite,insulating particles having conductive coatings, metals such as silver,gold, palladium, and aluminum, and alloys of such metals, andcombinations thereof.
 3. The composition of claim 1 wherein theconductive material in durable electrical contact with the inorganicparticles and the conductive material dispersed in the elastomericmaterial are different materials.
 4. The composition of claim 1 whereinthe combined amount of conductive material in the composition is betweenabout 40 and about 70 vol % of the amount of conductive material neededto attain the composition's percolation threshold.
 5. The composition ofclaim 1 further comprising a relative dielectric constant value thatchanges in a non-linear manner upon a change in applied voltage.
 6. Thecomposition of claim 1 wherein the volume loading of the inorganicparticles in the composition is about 20 to about 40 volume percent. 7.The composition of claim 1 wherein the elastomeric material comprisessilicone.
 8. The composition of claim 1 wherein the filler materialcomprises nanosilica-modified barium titanate.
 9. The composition ofclaim 9 wherein the conductive material comprises carbon.
 10. Thecomposition of claim 1 further comprising silicone oil.